Important deposits of high quality kaolin in the Czech republic
(sedimentary rock, with a significant proportion of kaolinite and other
clay minerals) are in Karlovy Vary region. A more extensive deposits of
kaolin are in the region of Horni Briza, Kaznejov, Chotikov and other
places in the northern neighborhood of Pilsen. Low-quality kaolin
deposits are situated in western Bohemia and also in Znojmo district.
Kaolin is widely used in the paper industry for its high whiteness and
low abrasion and it is also used in the other industry branches like
construction, ceramics, rubber and porcelain manufacturing.

The empirical formula for kaolinite is
[Al.sub.2][Si.sub.2][O.sub.5][(OH).sub.4] and the theoretical chemical
composition is Si[O.sub.2], 46.54 %; [M.sub.2] [O.sub.3], 39.50 %; and
[H.sub.2]O, 13.96 %. Kaolinite has a 1:1 sheet structure consisted by
tetrahedral and octahedral sheets. Type 1:1 minerals generally do not
have the charge or its value is extremely low (Murray, 2007). The
kaolinite layered structure 1:1 according to (Kuhn and Zamarsky, 1984)
is shown in Figure 1.

The quality respectively the orderliness of kaolinites strongly
influence their physical and chemical properties. The most often used
method for examination of the degree of structural disorder or
"crystallinity" of the kaolinite samples is X-ray powder
diffraction. Kaolinite tetrahedral and octahedral network forms a single
layer with a thickness of approximately 7 [Angstorm]. The poor
structural order commonly observed in kaolinites can be explained in
terms of a series of stacking faults or defects in the ab plane and
along the c-axis. The XRD patterns of ordered kaolinite samples are
significantly different from those of disorder. Ordered kaolinite shows
sharp and narrow peaks, while its disordered counterpart gives less
well-defined, broad, and asymmetrical peaks (Brigatti et al., 2006). The
crystallinity indices given by many authors are widely used for
determination of kaolinite disorder degree using various approaches
based on X-ray diffraction (Brindley et al., 1963; Plancon and Zacharie,
1990; Aparicio and Galan, 1999; Chmielova and Weiss, 2002; Aparicio,
2006).

Less common used method for determination of kaolinite disorder
degree is an infrared spectroscopy. The structural disorder of
kaolinites can be detected by differences in position and relative
intensity of OH stretching and bending bands in IR spectrum (Brindley,
et al., 1986; Muller and Bocquier, 1987; Prost et al., 1989; Madejova et
al., 1997). On the basis of change in the relative intensities of
absorption bands corresponding to the stretching and bending vibrations
of structural OH groups, it was possible to divide the analyzed samples
into three groups: with ordered structure, partially ordered structure
and poorly ordered structure.

[FIGURE 1 OMITTED]

The least common used method for determination of kaolinite
disorder degree is simultaneous thermogravimetry and differential
thermal analysis. The DTA curves show the endothermic and exothermic
reactions in a kaolinite sample during heating, such as desorption of
surface water ([H.sub.2]O), dehydroxilation (structural OH-groups) and
the transformation to mullite and cristobalite (Foldvari, 1997;
Hatakeyama and Liu, 1998; Guggenheim and Koster van Groos, 2001). The
obtained temperature effects of dehydroxilation and polymorphic
transformation are strongly dependent on kaolinite sample structural
order (Smykatz-Kloss, 1974; Kristof et al., 2002).

In the present paper is incorporated a detailed study of the degree
of structural disorder for four Czech kaolinite samples (Jimlikov,
Sedlec, Olomucany and Unanov deposits) and one kaolinite standard KGa-1b
from Georgia deposit obtained from the Source Clays Repository of The
Clay Minerals Society (USA). The kaolinite standard KGa-1b is the aim of
the interest of many authors because of its purity and its special
properties (Pruett and Webb, 1993; Bereznitski et al., 1998;
Sanchez-Soto et al., 2000; Mermut and Cano, 2001; Traore, 2006). This
standard kaolinite was chosen for comparision with Czech selected
kaolinites mentioned above. The detailed study has included mainly an
numerical approach based on crystallinity indices [CI.sub.1] and
[CI.sub.2] calculated from the intensities of selected vibrations modes
obtained by FTIR measurements as well as the determination of the
disorder degree according to peak temperature values of kaolinite
dehydroxilation and transformation obtained by differential thermal
analysis.

2. SAMPLES AND EXPERIMENTAL METHODS 2.1. TESTED SAMPLES

Kaolinite samples (fraction under 5[micro]m) from several different
Czech kaolin deposit were used for analysis:

Following Table 1 shows the volume of kaolinite and type of
associated minerals signed as : Q--quartz, I--illite, Sm--smectite,
D--dickite, F--feldspar.

2.2. METHOD OF FTIR SPECTROSCOPY

The infrared spectra were recorded on Nicolet Avatar 320 FTIR
spectrometer. The KBr pressed-disc technique was used for routine
scanning of the spectra. Samples of 2 mg and 0.5 mg were dispersed in
200 mg of KBr to record optimal spectra in the regions of 4000-3000 and
4000-400 [cm.sup.-1] with a resolution of 4 [cm.sup.-1] and 64 scans.
Discs for the 4000-3000 [cm.sup.-1] region were heated in the furnace
overnight at 150[degrees]C to minimize the water adsorbed on KBr and
clay sample (Madejova et al., 1997).

Two approaches were used to determine the degree of structural
disorder of selected kaolinites from IR spectra. The firstly step, an
empirical approach (IR-E) on the basis of resolution and relative
intensities of the bands in OH stretching and bending region was used.
The samples were classified as:

* ordered (well-ordered) if the OH stretching and bending bands
were clearly resolved;

* partially ordered if the individual OH bands at 3670, 3650 and
938 [cm.sup.-1] could be identified but their intensities were low; and

* poorly ordered if only one band near 3660 or inflexions near
3670, 3650 and 938 [cm.sup.-1] were observed in the spectra (Madejova et
al., 1997).

A numerical approach (IR-N) based on crystallinity indices
[CI.sub.1] and [CI.sub.2] calculated from the intensities of selected
vibrations modes using the equations:

[CI.sub.1] = I([V.sub.1]) / I([V.sub.3]) (1)

[CI.sub.2] = I([V.sub.4]) / I([V.sub.1]) (2)

where I([V.sub.1]) and I([V.sub.4]) are intensities of the OH
stretching bands at 3695 [cm.sup.-1] and 3620 [cm.sup.-1] and
I([V.sup.3]) is the intensity of the OH bending band at 915 [cm.sup.-1].
The two-points baseline method was used to obtained the intensity of the
OH bands. According to the obtained values of crystallinity indices
kaolinites were classified as poorly ordered structures ([CI.sub.1] <
0.7, [CI.sub.2] > 1.2); partially ordered structures (0.7 <
[CI.sub.1] < 0.8, 0.9 < [CI.sub.2] < 1.2) and ordered
structures ([CI.sub.1] > 0.8, [CI.sub.2] < 0.9) (Russell and
Fraser, 1994; Madejova and Komadel, 2001).

2.3. METHOD OF THERMAL ANALYSIS

Simulatneous thermogravimetry and differential thermal analysis was
carried out using multimodular thermal analyzer SETSYS 12-SETARAM
instrument equipped with a measurement head TG ATD ROD according to
below mentioned conditions. Measurement of each sample was carried five
times to get predicative results.

Based on obtained DTA curves, respectively according to
decomposition peak temperatures is possible to determine the degree of
disorder in kaolinite samples. Well ordered sample has their
decomposition peak temperature of quite high value ([T.sub.d] >
571[degrees]C), ordered sample has their decomposition peak temperature
[T.sub.d] at the temperature interval 561-570 [degrees]C. For poorly
ordered sample, the temperature decomposition varies from 546 to
560[degrees]C and disordered sample decomposition temperature values is
up to 545[degrees]C. The decomposition peak below 540[degrees]C seems to
be quite low according to published DTA data of kaolinites and belog to
only a very few naturally occuring, extremely disordered samples.

The differentiation mentioned above can be used for the
determination of the disorder degree of kaolinites if the analysis were
under the proposed conditions : heating rate 10[degrees]C.[min.sup.-1] ,
sample fraction under 5 [micro]m (the fraction from 0.5 to 5 [micro]m
with maximum in their grain size distribution curves laying for the
fraction 2-3 [micro]m) , sample mass about 50 mg, thermocouple
Pt-[Pt.sub.90] /[Rh.sub.10] , inert material [Al.sub.2][O.sub.3], no
pressing in crucible (loose packed), air atmosphere. With different
conditions, the dehydroxilation peak temperature [T.sub.d] can vary, so
the temperature interval of disorder degree can differ too. The most
influencing factor for the [T.sub.d] temperature value is the heating
rate (Gabbott, 2008). The difference between the dehydroxilation
temperature with heating rates 3[degrees]C.[min.sup.-1] and
30[degrees]C.[min.sup.-1] for the same kaolinite sample is over
50[degrees]C. Therefore the evaluation of the disorder degree of
kaolinites by thermal analysis must be always adjusted to measurement
conditions (Smykatz-Kloss, 1974).

3. RESULTS AND DISCUSSION

3.1. FTIR SPECTROSCOPY

The infrared spectra of tested kaolinites are shown in Figures 2
and 3. The complete assignment of the absorption bands in measured IR
spectra is summarized in Table 2.

Determination of degree of structural disorder from IR spectra of
kaolinites based on visual estimation of specific features (IR-E) and
calculated crystallinity indices [CI.sub.1] and [CI.sub.2] (IR-N)
according to Neal and Worral (1977) are given in Table 3.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The 3695, 3620 [cm.sup.-1] doublet is characteristic for the kaolin
group general. Four clearly resolved absorption bands at about 3695,
3670, 3650 and 3620 [cm.sup.-1] reflect high structural ordering of the
samples. Distinguishable four bands in OH stretching region were
observed in IR spectrum of sample KGa-1b, Jimlikov, Sedlec and Unanov.
The IR spectrum of kaolinite from Olomucany deposit showed the
replacement of the middle two bands by a single broad band at 3654
[cm.sup.-1] with a shoulder near 3668 [cm.sup.-1], that indicates
partially ordered structure of this sample (Fig. 3). IR spectrum of
Unanov demonstrated general broadening of all bands (Fig. 2). This
feature confirms high degree of disordering in kaolinite. Two clearly
resolved OH bending bands occuring at about 938 and 916 [cm.sup.-1]were
observed in IR spectra of samples from Georgia (KGa-1b) and Jimlikov
deposit (Fig. 2). Other samples in this spectral region showed only a
very weak inflection located at 934 [cm.sup.-1]. In all IR spectra were
found two weak bands at 795 and 758 [cm.sup.-1] with about equal
intensity (Fig. 2). This attribute can help to distinguish between
well-crystallized kaolinite and halloysite.

It is possible to divide tested kaolinites according to visual
estimation of degree of crystallinity from IR spectra (IR-E) into three
groups: ordered (well ordered), partially ordered and poorly ordered.
First group is represented by kaolinite KGa-1b and Jimlikov whose IR
spectra satisfy all the demands of IR-E classification. IR spectrum of
Sedlec kaolinite without clearly resolved OH deformation band at 938
[cm.sup.-1]belongs to the second group with partial structural order.
Kaolinite Olomucany and Unanov represent third group with poorly order
structure. According to calculated values of IR-N classification, based
on the ratio hydroxyl band absorbances 3695 and 915
[cm.sup.-1]([CI.sub.1]); and 3695 [cm.sup.-1] ([CI.sub.2]), it is
possible to divide the kaolinites into two groups: ordered (well
ordered) and partially ordered. Among ordered (well ordered) kaolinites
belong KGa-1b and Jimlikov. In this case data from IR-E classification
correspond well with IR-N classification. Second group is represented by
kaolinite Sedlec, Olomucany and Unanov, whose CI1 and CI2 crystallinity
indices corresponded to partially ordered structures. Discrepancy
between IRN and IR-E classification arose probably thank to admixture of
fine-grained illite or smectite present in kaolinite samples (see Table
1). Both minerals (illite and smectite) have their absorption bands at
the same area and can contribute to the increasing of both OH bending
absorption at 915 [cm.sup.-1]and OH stretching absorption at 3695
[cm.sup.-1]. The problem is that their contribution to the absorption at
915 [cm.sup.-1]is significantly higher than to the absorption at 3695
[cm.sup.-1]. When calculating the ratio of [CI.sub.2] based on such
affected values, the ratio can indicate an erroneously higher value for
the crystallinity degree of the kaolinite.

3.2. THERMAL ANALYSIS

The measured DTA curves of tested kaolinites showed the following
endothermic effects: desorption of surface water at low temperature
interval 105110 [degrees]C and releasing of constitution water and
breaking of crystal lattice at higher temperature interval
552-576[degrees]C. The recrystallization and transformation of
dehydrated substance to mullite, cristobalite and quartz was observed as
the exothermic effect at about 982-1001[degrees]C.

Table 4 shows the thermal data obtained from DTA curves including
dehydroxilation peak temperature [T.sub.d] and transformation peak
temperature [T.sub.t]. The first group of well ordered samples is
represented by kaolinite KGa-1b with the highest decomposition peak
temperature value (above 571[degrees]C). The sample classified as
ordered (from Jimlikov) belongs to the second group with middle
decomposition peak temperature values. Among poorly-ordered samples in
the third group belong kaolinite samples from Sedlec, Olomucany and
Unanov with the lowest decomposition peak temperature values up to
560[degrees]C.

From obtained data is evident, that with increasing of
decomposition peak temperature value, respectively with increasing of
orderliness, the transformation exothermic peak temperature also
increase (Fig. 4). DTA curves show, that for well ordered samples, the
transformation exothermic peak are of the high intensity and quite
narrow. For the poorly ordered sample, the transformation peaks are of
low intesity, wider and smaller.

[FIGURE 4 OMITTED]

The kinetic parameters of decomposition was studied using the
Arrhenius equation applied to solid state reactions. The activation
energy for dehydroxilation was evaluated by Freeman-Carroll method which
enables, from the exploitation of one DTG peak, to determine besides
activation energy of reaction also the reaction order and the rate
constant (Freeman and Caroll, 1958). Table 5 summarized thermal data
including maximal reaction velocity temperature [T.sub.r], activation
energy [E.sub.a] with order reaction n and correlation factor [R.sup.2].

It is evident, that values of maximal reaction velocity temperature
[T.sub.r] obtained from DTG curve correspond to the decomposition peak
temperature [T.sub.d] obtained from DTA curves. Figure 5 shows the
derivative thermogravimetric (DTG) curves for all tested kaolinite
samples at the heating rate 10[degrees]C[.min.sup.-1]. With increase of
peak temperature on DTG curve, activation energy of dehydroxilation
process decreased. The values of activation energy [E.sub.a] varied from
230 kJ.[mol.sup.-1] (well ordered kaolinite sample KGa-1b) to 301
kJ.[mol.sup.-1] (the poorliest ordered sample Unanov) with the reaction
order n varied from 0.9 to 1.5. Obtained results show, that better
ordered samples display lower activation energy for dehydroxilation
process estimated by Freeman-Carroll method. The possibilities and
comparision of Freeman-Carroll method with other used methods are
described well by (Slovak, 2001). The disadvantage of this method can
lay in the fact, that the calculation is performed from the difference
of two discrete experimental points and consider rather homogenous
character of reaction. MacKenzie (1978) and Cicel et al. (1981) assume,
that homogeneous and inhomogeneous mechanisms of the dehydroxilation
process is possible. Therefore if the heterogenous character of
dehydraxilation process is supposed, the dependance of activation energy
on structural order can differ, mainly if we cosider that the
dehydroxilation process is initiated in the defect places.

[FIGURE 5 OMITTED]

4. CONCLUSIONS

The obtained results from infrared spectroscopy and differential
thermal analysis were compared with XRD results previously published by
Hinckley (1963) and Aparicio et al. (1999). Also according to Pruett and
Webb (1993), kaolinite Georgia (KGa-1b) is signed as well-ordered and
based on the calculated characteristic (Chmielova and Weiss, 2002), the
kaolinites from Czech deposits can be divided into two groups. First
group of medium-degree of structural order (ordered) samples contains
kaolinite Jimlikov, the second group of low-degree of structural order
(poorly ordered) samples contains kaolinites from Sedlec, Olomucany and
Unanov deposits. According to our DTA results, kaolinite Georgia
(KGa-1b) belongs to well-ordered group, kaolinite Jimlikov to ordered
group and other tested samples to poorly-ordered group. It is evident,
that DTA data set a reasonable agreement with published data obtained by
XRD diffraction method. In case of using FTIR spectroscopy, there is
only a slight difference between IR data and data evaluated by DTA and
XRD analysis. The OH stretching and bending vibrations of structural
water are sensitive enough to distinguish ordered (well ordered),
partially ordered and poorly ordered kaolinites. Visual IR-E method for
structural order determination provides basic classification into three
groups: ordered (well ordered), partially ordered and poorly ordered;
while IR-N method divides kaolinite samples into two groups: ordered
(well ordered) and partially ordered. Unanov and Olomucany samples were
evaluated as partially ordered kaolinite analogous to Sedlec. The cause
of this discrepancy is probably the presence of clay mineral admixture
(e.g. illite or smectite), which strongly influences the IR pattern in
OH stretching and bending region. The OH bands are overlapped and thus
may lead to incorrect deegre of structural order. But even though all
three used methods give a slightly different distribution of the
structure classification, the order respectively the resulting trend of
decrease in orderliness is the same for all three methods: KGA-1b (with
the best orderliness) > Jimlikov > Sedlec > Olomucany >
Unanov (with the worst orderliness).

ACKNOWLEDGEMENTS

This study was supported by the Czech Science Foundation, the
project No.105/08/1398 and No.105/07/P416, and by Research plan No. AVOZ
30860518. The authors would like to thank George Laynr for controlling
and correcting the use of English in this article.